Brushless direct current (BLDC) motors are utilized in many applications, particularly devices such as computer hard drives and CD/DVD players. Cooling fans in electronic equipment are powered by BLDC motors. They are also found in cordless power tools. Low speed and low power BLDC motors are used in direct-drive turntables for analog audio discs. High power BLDC motors are found in electric vehicles and hybrid vehicles.
Electric vehicles operate over wide ranges of speed and load. The motor of the vehicle must have sufficient torque to initially move the vehicle at a low speed, and must also be capable of propelling the vehicle at a desired cruise speed. For direct current (DC) motors, the available supply voltage and the designed maximum motor speed effectively determine the motor voltage constant (volts/rad/s), and hence, the motor torque constant (N-M/amp). As vehicle design cruise speeds increase, there must be a corresponding decrease in the voltage and torque constants in order to remain within the voltage limit of the power supply. The result is that for a fixed supply voltage, starting torque is reduced as vehicle speed capability is increased.
Some conventional motors employ a mechanical transmission with two or more speeds. However, a mechanical transmission adds weight, cost and efficiency losses to the vehicle power train. Alternate conventional motors provide a BLDC motor with adjustable voltage and torque constants. Stator windings are switched between various combinations of serial and parallel connections on a common stator.